usp:s of natural areas 



39 



River the same condition prevails in 

 many places (Leighton, 1904). Such 

 pollution causes a complete extermina- 

 tion of the fauna (and largely of the 

 chlorophyl-bearing flora) and leaves 

 the stream in such condition that re- 

 stocking by either natural or artificial 

 means is practically impossible, and 

 if attempted is a waste of money. 



Pollution by sewage, when the pol- 

 luting material is of small percentage 

 as compared with the pure water of 

 the stream (as 200 to 1), causes little 

 inconvenience to the animal life and 

 is doubtless of some benefit because of 

 the additional food material that is 

 added (Forbes and Richardson, 1919, 

 p. 146). But the streams seldom remain 

 long in this innoxious condition, the 

 sewage becoming more and more concen- 

 trated and less diluted until the whole 

 stream may be supersaturated with 

 noxious substances, the amount of 

 oxygen in saturation reduced, and 

 the biota finally driven out or killed. 



Pollution is worst and usually most 

 deadly to animal life during periods of 

 low water and in winter when the 

 amount of water in the stream is small 

 and the decomposing organic material 

 has less water to deprive of its dis- 

 solved oxygen. During times of floods 

 the putrescent material is also carried 

 down stream for many miles and con- 

 taminates areas not previously af- 

 fected. 



While all clean-water forms of ani- 

 mal life are more or less affected by 

 sewage pollution, the decomposition of 

 the organic matter abstracting dissolved 

 oxygen from the water and rendering 

 it unsuitable for aquatic life, the fish, 

 river mussels, and crayfish are partic- 

 ularly affected, most fish being es- 

 pecially sensitive to contaminated water. 

 Some fish (as the brook silversides, 

 Labidesthes sicculus) are notably sensi- 

 tive, while others (as the black bull- 

 head, Ameiurus melas) will endure 

 water that is badly polluted (Shelford, 

 1918, p. 27; Wells, 1918, pp. 562 567). 

 Young fish are relatively more sensitive 

 than adult fish. It is noteworthy that 



the more resistant species of fish arc 

 inhabitants of sluggish Ijodics of water, 

 as ponds and shallow lakes, while the 

 least resistant species live in running 

 streams. It seems to be a question 

 of the amount of oxygen necessary for 

 the well being of the fish. 



The ill effect of sewage pollution is 

 most marked on the bottom of bodies 

 of water, where a sludge is formed, 

 often of great thickness (as much as 

 10 ft. in several cases), consisting of 

 a mass of soft, black, sediment, with 

 a high content of organic matter, in 

 which only a few organisms, normally 

 inhabitants of polluted streams, can 

 live (e.g., septic Protozoa and Rotifera, 

 foul-water algae, and slime-worms, 

 Tubificidae). This effect on the bot- 

 tom is perhaps the most serious phase 

 of stream pollution because the septic 

 condition of this area continues in 

 operation long after the original source 

 of contamination ceases to operate. 

 This sludge formation renders the 

 bottom unfit for clean-water life upon 

 which many fish depend for food. The 

 time necessary for the recovery of the 

 normal biota of such a stream will in 

 most cases be of long duration, and in 

 the case of a stream polluted with 

 wastes from mines and chemical nianu- 

 facturies, there may never be a return 

 to the original condition. 



In the case of the Genesee River at 

 Rochester, N. Y., we have a striking 

 example of the history of a polluted 

 stream and its effect on one group of 

 the animal life. Previous to the stage 

 of great pollution there is a varied 

 fauna of moUusks very numerous in 

 individuals. In the course of 11 

 years the gill-bearing species are forced 

 out and after a lapse of 14 years 

 all moUuscan life ceases to live in this 

 part of the river. Seven years later 

 the greater amount of sewage is diverted 

 to another outlet. Two years after 

 this change we find that the mollusks 

 have returned in as great numbers as 

 before the maximum stage of pollution. 

 The significance of all this lies in the 

 fact of the early return of this life, and 



